Java：日期类Date与Calendar

Timestamp类型与日期类型之间的转化？

            Timestamp timestamp = Timestamp.valueOf("2017-03-17 07:00:00");
            Date date = new Date(timestamp.getTime());
            SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            System.out.println(format.format(date));

Java如何字符串日期转化为日期类型？

    /**
     * 字符串转换成日期
     *
     * @param str
     * @return date
     */
    public static Date strToDate(String str) {
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date date = null;
        try {
            date = format.parse(str);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return date;
    }

怎么获取系统系统时间，通过java?

Date now=   new Date();
SimpleDateFormat dateFormat=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSSSSSSSS");
String nowFormate=   dateFormat.format(now);
System.out.println(nowFormate);

输出结果

2017-02-16 20:56:12.000000278

如何获取当前系统是星期？

方法一、

    String weekOfDay= getWeekOfDay(now);
    System.out.println(weekOfDay);

    public static String getWeekOfDay(Date date){
        String[] weekDays=new String[]{"星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六"};
        Calendar calendar= Calendar.getInstance();
        calendar.setTime(date);
       int weekIndex= calendar.get(Calendar.DAY_OF_WEEK)-1;
        if(weekIndex<0)
            weekIndex=0;

        return weekDays[weekIndex];
    }

方法二、

        SimpleDateFormat   dateFormat2=new SimpleDateFormat("EEEE");
        String nowFormate2=   dateFormat2.format(now);
        System.out.println(nowFormate2);

Calendar获取当前系统日期的年、月、日

        Calendar calendar= Calendar.getInstance();
        calendar.setTime(now);
        int year= calendar.get(Calendar.YEAR);
        int month= calendar.get(Calendar.MONTH)+1;
        int day= calendar.get(Calendar.DAY_OF_MONTH);
        System.out.println(year+"-"+month+"-"+day);